This invention relates to an improved metal carbide heat source and an improved method for making the heat source. The method and heat source of this invention are particularly suitable for use in a smoking article such as that described in commonly assigned U.S. Pat. No. 4,991,606. The heat source made by the method of this invention has low ignition and high combustion temperatures that generate sufficient heat to release a flavored aerosol from a flavor bed for inhalation by the smoker. Upon combustion, the heat sources of this invention produce substantially no carbon monoxide.
According to the method of this invention, a metal species is mixed with a carbon source and converted to metal carbide by heating the mixture in the presence of a carbidizing atmosphere. In a preferred embodiment, the metal species/carbon source mixture is pre-formed into a desired shape and converted to metal carbide in situ, without substantially altering the shape of the mixture.
There have been previous attempts to provide a heat source for a smoking article. While providing a heat source, these attempts have not produced a heat source having all of the advantages of the present invention.
For example, Siegel U.S. Pat. No. 2,907,686 discloses a charcoal rod coated with a concentrated sugar solution which forms an impervious layer during burning. It was thought that this layer would contain gases formed during smoking and concentrate the heat thus formed.
Ellis et al. U.S. Pat. No. 3,258,015 and Ellis et al. U.S. Pat. No. 3,356,094 disclose a smoking device comprising a nicotine source and a tobacco heat source.
Boyd et al. U.S. Pat. No. 3,943,941 discloses a tobacco substitute which consists of a fuel and at least one volatile substance impregnating the fuel. The fuel consists essentially of combustible, flexible and self-coherent fibers made of a carbonaceous materials containing at least 80% carbon by weight. The carbon is the product of the controlled pyrolysis of a cellulose-based fiber containing only carbon, hydrogen and oxygen.
Bolt et al. U.S. Pat. No. 4,340,072 discloses an annular fuel rod extruded or molded from tobacco, a tobacco substitute, a mixture of tobacco substitute and carbon, other combustible materials such as wood pulp, straw and heat-treated cellulose or a sodium carboxymethylcellulose (SCMC) and carbon mixture.
Shelar et al. U.S. Pat. No. 4,708,151 discloses a pipe with replaceable cartridge having a carbonaceous fuel source. The fuel source comprises at least 60-70% carbon, and most preferably 80% or more carbon, and is made by pyrolysis or carbonization of cellulosic materials such as wood, cotton, rayon, tobacco, coconut, paper and the like.
Banerjee et al. U.S. Pat. No. 4,714,082 discloses a combustible fuel element having a density greater than 0.5 g/cc. The fuel element consists of comminuted or reconstituted tobacco and/or a tobacco substitute, and preferably contains 20%-40% by weight of carbon.
Published European patent application 0 117 355 by Hearn et al. discloses a carbon heat source formed from pyrolized tobacco or other carbonaceous material such as peanut shells, coffee bean shells, paper, cardboard, bamboo, or oak leaves.
Published European patent application 0 236 992 by Farrier et al. discloses a carbon fuel element and process for producing the carbon fuel element. The carbon fuel element contains carbon powder, a binder and other additional ingredients, and consists of between 60 and 70% by weight of carbon.
Published European patent application 0 245 732 by White et al. discloses a dual burn rate carbonaceous fuel element which utilizes a fast burning segment and a slow burning segment containing carbon materials of varying density.
These heat sources are deficient because they provide unsatisfactory heat transfer to the flavor bed, resulting in an unsatisfactory smoking article, i.e., one which fails to simulate the flavor, fuel and number of puffs of a conventional cigarette.
Commonly assigned U.S. Pat. No. 5,076,296 and commonly assigned herewith, solved this problem by providing a carbonaceous heat source formed from charcoal that maximizes heat transfer to the flavor bed, releasing a flavored aerosol from the flavor bed for inhalation by the smoker, while minimizing the amount of carbon monoxide produced.
However, all conventional carbonaceous heat sources liberate some amount of carbon monoxide gas upon ignition. Moreover, the carbon contained in these heat sources has a relatively high ignition temperature, making ignition of conventional carbonaceous heat sources difficult under normal lighting conditions for a conventional cigarette.
Attempts have been made to produce noncombustible heat sources for smoking articles in which heat is generated electrically. E.g., Burruss, Jr., U.S. Pat. No. 4,303,083, Burruss U.S. Pat. No. 4,141,369, Gilbert U.S. Pat. No. 3,200,819, McCormick U.S. Pat. No. 2,104,266 and Wyss et al. U.S. Pat. No. 1,771,366. These devices are impractical and none has met with any commercial success.
Attempts have been made to produce a combustible, non-carbonaceous heat source. Commonly assigned U.S. Pat. No. 5,040,522 relates to a metal carbide heat source. Although combustion of the metal carbide heat source yields up to tenfold less carbon monoxide than combustion of conventional carbonaceous heat sources, some carbon monoxide is still produced. Copending U.S. patent application Ser. No. 443,636, filed on Nov. 29, 1989, and commonly assigned herewith, relates to a metal nitride heat source that also produces substantially no carbon monoxide or nitrogen oxides upon combustion. Copending U.S. patent application Ser. No. 556,732, filed on Jul. 20, 1990, and commonly assigned herewith is directed to a heat source comprising carbon and metal carbide that also produces substantially no carbon monoxide upon combustion.
Attempts have been made to produce pyrophoric materials comprising metal aluminides for use as a decoy for heat-seeking missiles. E.g., Baldi, U.S. Pat. No. 4,799,979. These devices, however, combust too rapidly and produce too intense a heat to be used as a heat source in a smoking article.
Methods of producing metal carbides are Grey et al. U.S. Pat. No. 3,885,023 and Okamura et al. published European patent application 0 180 162 disclose the preparation of iron carbide particles by reducing iron oxide in a carbon monoxide atmosphere. Stelling et al. U.S. Pat. No. 2,780,537, Okamura U.S. Pat. No. 4,842,759 and Shibuya et al. published European patent application 0 123 318 disclose the preparation of iron carbide particles by reducing iron oxide in a carbon monoxide/reducing gas mixture. Rogers U.S. Pat. No. 3,572,993 discloses the preparation of ultrafine iron carbide particles by reducing iron carbonyl in a carbon monoxide/hydrogen atmosphere.
Additionally, metal carbides may be prepared by reduction of the metal oxide with elemental carbon, carbidization of the metal or metal oxide with a gaseous species such as methane, ethane, ethylene, or propane, and direct reaction of the fully reduced metal with elemental carbon (Goldschmidt, H. A., Interstitial Alloys, pp. 214-31 (Butterworth, London, 1967)).
A shortcoming of these production methods is the lack of control over end-product composition. The above methods yield polymorphous iron carbide containing carbon deposits which, upon combustion, incompletely oxidize to generate carbon monoxide, albeit at lower levels than in carbonaceous heat sources.
Yet another shortcoming of prior methods is the number of steps required to produce a metal carbide heat source. These steps involve mixing metal carbide precursor materials, reducing the precursor materials, carbidizing the intermediate metal species, mixing the metal carbide with a binder, forming the metal carbide/binder mixture into a desired shape, and baking the mixture.
A further shortcoming of known methods of preparing metal carbide is that the metal carbide is produced in particulate form. Iron carbides by nature are brittle, intractable materials, which, once formed, are difficult and expensive to form into a desired shape.
It would be desirable to provide a method for producing metal carbide which allows for control of end-product composition.
It would also be desirable to provide a method of producing a heat source comprising metal carbide that requires fewer steps than known methods.
It would be further desirable to provide a method of producing metal carbide in which the starting materials are pre-formed into a desired shape and converted in situ to metal carbide.
It would be desirable to provide a heat source that liberates virtually no carbon monoxide upon combustion.
It would also be desirable to provide a heat source that has a low ignition temperature to allow for easy lighting under conditions typical for a conventional cigarette, while at the same time having a combustion temperature high enough to provide sufficient heat to release flavors from a flavor bed.
It would further be desirable to provide a heat source that does not self-extinguish prematurely.
It would also be desirable to provide a heat source which is stable at ambient temperature and humidity.